JPH0369980B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-toughness seamless steel pipe that exhibits excellent toughness and good weldability without requiring special heat treatment after pipe manufacturing. In recent years, due to changes in the world's energy situation, large-scale oil and natural gas fields have been developed in areas with harsh weather conditions such as Alaska, Canada, and Siberia, and the demand for transportation steel pipes (line pipes) for these fields has increased significantly. However, these line pipes for cold regions are not only designed to withstand extreme cold, but also have higher strength and higher strength to improve transportation efficiency.
High toughness is now required. Incidentally, in order to manufacture seamless steel pipes, which occupy an important position as oil country tubular goods such as line pipes, the steps shown in Table 1 below are normally used. [Table] [Table] Conventionally, when seamless steel pipes manufactured in this way are required to have high toughness and higher strength, it is necessary to reduce the C content to the maximum possible level, although this varies depending on the type and grade. This is handled by maintaining the steel as high as possible, and then subjecting it to heat treatment such as quenching, tempering, or normalizing after pipe-making by adding appropriate amounts of Mn and other alloying elements while considering weldability, etc. It was normal. However, if such a heat treatment method is adopted to ensure toughness and strength, the cost of the seamless steel pipe product itself can be avoided due to an increase in the number of heat treatment steps and an increase in the amount of ferroalloy added to improve hardenability. Not only is it impossible to do so, but quenching and tempering results in large variations in product dimensional accuracy due to thermal effects.Also, in the normalizing method, many alloying elements are added to improve hardenability to ensure strength. However, the difficult problem of deterioration of weldability due to the use of these materials could not be avoided. On the other hand, in the manufacture of sheet materials, in order to achieve high strength and toughness as rolled, low-temperature finish rolling is performed just below the _A3 transformation point to prevent the formation of bainite and develop a fine ferrite/pearlite structure. However, when this controlled rolling method is applied to the production of seamless steel pipes using the Mannesmann plug mill method, due to the machining rate, the perforations shown in B and C in Table 1 are unavoidable. It is necessary to perform low temperature processing in the process, which is difficult to implement with conventional equipment due to the characteristics of the equipment, and if the processing temperature in this process is lowered, it will not be possible to secure the required temperature in the subsequent reeler and sizer finishing processes. There was a problem. From the above-mentioned viewpoints, the inventors of the present invention have developed a method that provides excellent toughness and strength, without requiring a post-tube-making heat treatment process that increases costs, and by using conventional pipe-making equipment as is. In order to manufacture seamless steel pipes with excellent weldability, we focused on the fact that the high-toughness, high-strength steel conventionally used for high-grade oil country tubular goods has a fine ferrite/pearlite structure. Recognizing that in order to manufacture seamless steel pipes with excellent toughness in the as-rolled state, it is necessary to transform the as-rolled steel structure into a fine ferrite/pearlite structure, we conducted various experiments and As a result of repeated research, we have developed a new method that has been used for line pipes to prevent the formation of bainite, which deteriorates the toughness of steel materials, depending on the rate of temperature drop in the normal seamless steel pipe manufacturing process. Steel C and
By keeping the Mn content low and compensating for the resulting decrease in strength by adding Cu and Ni, which are elements that strengthen the ferrite matrix, and adding Ti and Nb, which are carbide-forming elements, to strengthen the initial austenite grains. By refining the steel, delaying the precipitation of pro-eutectoid cementite during cooling, and producing fine ferrite with carbide as the nucleus, this steel can be made into pipes under conventional conditions, creating a ferrite-pearlite structure with a low pearlite occupation area ratio. This led to the discovery that it is possible to obtain seamless steel pipes with high toughness and high strength. and,
The steel pipe thus obtained was found to have good weldability, partly because the amount of alloying elements added was low. This invention has been made based on the above knowledge, and provides a seamless steel pipe with a chemical composition of C: 0.02 to 0.10% (hereinafter, % indicating the compositional component ratio is % by weight), Si: Contains 0.10 to 0.80%, Mn: 0.70 to 1.90%, Ni: 0.05 to 0.70%, Cu: 0.10 to 1.00%, and one or more of the following, with the formula 0.15Ni (%) + Cu (%) 1.00 It also contains one or more of the following: Ti: 0.01 to 0.05%, Nb: 0.02 to 0.07%, and Fe and unavoidable impurities: the remainder; By changing the original structure to a ferrite/pearlite structure with a pearlite occupation area ratio of 10% or less, we have achieved high strength and excellent toughness without using large amounts of alloying elements. It has characteristics. Next, the reason why the component composition and steel structure of the seamless steel pipe of the present invention are limited as described above will be explained. (a) Ingredient composition ○…

Claims (1)

[Claims] 1 Contains C: 0.02 to 0.10%, Si: 0.10 to 0.80%, Mn: 0.70 to 1.80%, Ni: 0.05 to 0.70%, Cu: 0.10 to 1.00%, of the following. Fe A high-toughness seamless steel pipe characterized by having a ferrite-pearlite structure consisting of (more than % by weight) and unavoidable impurities: the remainder, and having a pearlite occupation area ratio of 10% or less as rolled.